United States v. Kirby was a United States Supreme Court case that addressed the issue of whether a federal statute could be applied retroactively. The case involved a dispute between the United States and the heirs of a deceased man, John Kirby, who had been convicted of a federal crime in 1845. The heirs argued that the federal statute under which Kirby had been convicted had been repealed in 1847, and thus could not be applied retroactively to Kirby's conviction. The Supreme Court disagreed, ruling that the statute could be applied retroactively, and that the conviction was valid. The Court reasoned that the repeal of the statute did not affect the validity of the conviction, as the repeal was not intended to be retroactive. The Court further noted that the repeal of a statute does not necessarily invalidate any convictions that occurred prior to the repeal. The Court concluded that the conviction was valid, and that the heirs of John Kirby were not entitled to any relief. The decision in United States v. Kirby established the principle that a federal statute can be applied retroactively, and that the repeal of a statute does not necessarily invalidate any convictions that occurred prior to the repeal. This principle has been applied in numerous cases since, and is an important part of the legal landscape today.

In United States v. Kirby, the Supreme Court was tasked with determining whether a federal statute that imposed an additional tax on certain vessels violated the Constitution. The majority opinion held that Congress had the power to impose such taxes under its authority to regulate commerce among states and foreign nations. Justice Field dissented from this decision, arguing that Congress did not have the constitutional authority to pass such a law because it exceeded their enumerated powers as outlined in Article I of the Constitution. He argued further that if Congress could use its taxing power for any purpose they saw fit, then there would be no limit on their ability to raise revenue or interfere with state governments' rights and responsibilities over taxation matters. Furthermore, he argued that allowing such broad interpretation of congressional taxing power would lead to arbitrary government action and undermine individual liberty by giving too much discretion in how taxes were collected and spent without proper checks or balances from other branches of government.